A color printing apparatus uses three basic colors of cyan, magenta, and yellow. In inkjet color printing, ink particles of the three basic colors are put on top of one another so as to produce a desired color at a certain region on paper. Further, in the inkjet color printing, brightness or luminosity is also taken into account, and a black color is added to the three basic colors in order to improve appearance of the desired color. The printing that uses these four primary colors takes root. C indicates cyan, M magenta, Y yellow, and K black. CMYK indicates the four basic colors.
One of widely used printing apparatuses is a color printing apparatus that performs bi-directional printing, i.e., performs forward printing by moving a printing head forwards in the direction perpendicular to the direction of sending a printing medium (e.g., paper) and also performs backward printing by moving the printing head backwards in the direction perpendicular to the direction of sending the printing medium. This color printing apparatus improves printing speed and printed resolution. In the bi-directional printing, the head moves forwards to print one line on the printing medium, and the head moves backwards to print another line on the printing medium. In such bi-directional printing, the head need not be moved back to the start position each time one line is printed on the printing medium, so that there is an advantage in that the printing speed is high.
In some cases, a color tone or a hue obtained by the inkjet color printing varies due to the order of printed inks as well as the relative amount of respective inks that are put on the printing medium. For example, the color obtained by the order in which cyan C is first printed, and magenta M is then printed on this cyan C is different from the color obtained by the order in which magenta M is first printed, and cyan C is then printed on this magenta M. The difference of the hue caused by the order of colors adhering to the printing medium becomes a problem in the case of the bi-directional printing method using the printing head having respective inks arranged in a row.
An example of the printing head of the inkjet color printing apparatus is schematically shown in FIG. 1. A printing unit is configured such that black, cyan, magenta, and yellow are arranged in that order toward the forward printing direction in the main running direction as shown in FIG. 1. The arranged order and the number of colors are not limited to this example, and the different arranged order and the different number of colors are used in accordance with ink characteristics, the printing head design, and the like.
In the case of the printing head unit shown in FIG. 1, the inks of K, C, M, and Y are ejected from the printing head in the order of K, C, M, and Y in the forward printing. On the other hand, the inks are ejected in the order of Y, M, C, and K on the printing medium in the backward printing. However, coloring material fixing characteristics affect a color tone. That is, when a second ink is put on a first ink at the same position on a printing medium, the first color ink that first adheres to the printing medium dominates the second color ink that adheres to the first color ink on the printing medium after the first color ink is printed.
FIG. 2 shows the distribution of coloring materials inside the printing medium when two dye type inks are put on the same position on a printing medium. The first ink that is first put on the printing medium spreads in the printing medium more widely than the second ink that is put on the first ink on the printing medium after the first ink is printed. Accordingly, the difference in the fixing region between the coloring materials is generated. As a result, when two color inks (e.g., C and M, or M and Y) are put on the same point on the printing medium, the color ink that is first put so as to adhere to the printing medium is more dominant than the other color that is then put on the first put ink.
FIG. 3 shows the inside (cross section) of the printing medium when two pigment type inks are put on the same point on the printing medium. The first ink that is first put on the printing medium stays at the surface of the printing medium, and the second color that is then put on the first ink on the printing medium sinks inside (beneath the surface of) the printing medium, as shown in FIG. 3. As a result, the first ink that stays at the surface of the printing medium becomes stronger and more dominant than the second ink.
When the bi-directional printing is performed in order to improve a printing speed, it is necessary to take into account the hue change regarding two colors or three colors (e.g., C, M, and Y) put on the same point on the printing medium caused by the coloring material fixing characteristics related to the order in which two or more color inks are put on the same position on the printing medium. Particularly, in the case where the printing medium is greatly moved in the paper sending direction each time one forward printing or backward printing is completed in the main running directions, there is a possibility that the hue change appears as a lateral stripe.
According to “a bi-directional inkjet color printing method” disclosed in Japanese Laid-Open Patent Application No. 11-320926, a printing head used for the forward printing, and a printing head used for the backward printing are employed so that the forward printing and the backward printing can be carried out in the same order of colors. In this method, the difference of the hue is suppressed, but manufacturing costs of the printing heads increase.
According to “a printing method in a forward and backward printing printer” disclosed in Japanese Published Patent Application No. 7-29423, in the printing operation, printing lines printed by the forward printing and printing lines printed by the backward printing are alternately arranged. In this method, the difference of the hue is suppressed by using the trick of the human eyesight, but when a printing job is done, the head always needs to be moved forwards and backwards, alternately. Accordingly, uni-directional printing cannot be performed, so that it is difficult to print an image of high quality that has no difference in the hue caused by the difference of the order of colors.
According to “a bi-directional color printing method in a liquid ink printer” disclosed in Japanese Laid-Open Patent Application No. 8-281976, a lookup table for the forward printing, and a lookup table for the backward printing are used to suppress the difference of the hue. However, a color calibration test sheet needs to be printed out in both the forward printing and the backward printing, the printed result on the color calibration test sheet needs to be measured by a scanner, a spectrophotometer, or a densitometer, and a color space conversion lookup table needs to be created. Furthermore, another lookup table becomes necessary each time a printing medium is changed. Accordingly, a burden on a user of the printer becomes large.
Furthermore, since an inkjet printing apparatus is cheap, and can produce an image having high quality by using specific paper, the inkjet printing apparatus is rapidly and widely used by individuals. In addition, recently, the inkjet printing apparatus as a printer capable of performing color printing took root in offices where electrophographic laser printers are mainly used. In order to further spread the inkjet printing apparatuses in offices, the two following big problems need to be solved.
First, the inkjet printing apparatus needs to use standard paper. In the case of using special paper, the inkjet printing apparatus can reproduce an image having very high quality. In recent years, the inkjet printing apparatus for use by individuals has become able to print an image of so high quality that one can take it for a photograph. However, the special paper that the inkjet printing apparatus uses for high quality printing is generally expensive, and it is difficult to introduce such an inkjet printing apparatus to companies that require strict cost management. Further, demerits are larger than advantages when the inkjet printing apparatus that can use only the special paper is used in the offices where high quality image characteristics are not frequently required.
The composition of ink has been improved in order to apply the standard paper to the inkjet printing apparatus. For example, development of dye type ink having low penetration in paper, use of a fixing assist agent, and development of pigment type ink have been tried. As a result, even when using the standard paper generally used in the offices such as copy paper, the recent inkjet printing apparatus can perform printing having the quality that is not inferior to the quality achieved by the laser printer.
The second problem is a printing speed. Except for a particular type inkjet printing apparatus such as an industrially used inkjet printing apparatus, when the inkjet printing apparatus performs printing, the printing head that is much smaller than the paper moves on the paper many times, and ejects ink on the paper. In this printing method, the printing is performed line by line. Compared with the electrophotographic printing method in which printing is performed page by page, i.e., surface by surface, the printing method by the inkjet printing apparatus is substantially disadvantageous in terms of the printing speed. In order to solve the disadvantage of the printing speed in the inkjet printing apparatus, a period of ejecting ink was shortened to improve speed of the printing head, the printing head was made large to decrease the number of the movements of the printing head, bi-directional printing was performed to decrease the number of the movements of the printing head, or a running sequence of the printing head was made more efficient or optimized to move the printing head to only the positions where the image data are printed. Accordingly, some of the most recent inkjet printing apparatuses can print the small-to-medium number of pages at a speed substantially equal to or higher than a speed of the electrophotographic printing method.
These improvements in the image quality and the printing speed made the inkjet printing apparatus more popular even in the offices. Particularly, the inkjet printing apparatus is more advantageous than the laser printer in terms of a cost, and the size of the inkjet printing apparatus is easily made small to promote the desktop usage of the inkjet printing apparatus.
In the printing by the laser printer or offset printing, coloring materials are fixed at the paper surface. However, in the printing by the inkjet printing apparatus, coloring materials are fixed after the coloring materials penetrate in the paper. The problem and the condition related to the ink penetration in the paper always accompany the printing by the inkjet printing apparatus.
In the printing performed by the inkjet printing apparatus, there is also the following problem. Water contained in the ink causes the paper to expand and be wet, and there is a case where the deformed paper contacts with the printing head, ink on the paper adheres to the printing head by this contact, and this ink adhering to the printing head is transferred to the paper (the secondary transfer occurs), resulting in the degraded image printed on the paper. It is desired to reduce a gap between the printing head and the paper as much as possible in order to raise the accuracy in the position where ink is ejected on the paper, but the standard paper used in the office is not suitable for the printing that causes the paper to expand and be wet. Accordingly, when the gap between the printing head and the paper is strictly made small, there is a possibility that the inkjet printing apparatus cannot perform printing on the standard paper without generating problems. Usually, there is a time lag between the time the ink adheres on the paper and the time the paper starts to expand. Furthermore, in offices, the printing speed often has priority over the image quality. Accordingly, as in the use in the offices, with the image quality being sacrificed to a certain degree, this problem can be neglected while the printing speed is raised.
Furthermore, as described above, in the bi-directional printing by the inkjet printing apparatus, a hue produced by the forward printing becomes different from a hue produced by the backward printing because a first color ink that is first printed on paper contributes to a hue larger than another color ink that is then printed on the paper at the same position of the first color ink, and the printing order of the color inks in the forward printing are reversed in the backward printing. Accordingly, a hue generated in a band or line printed in the forward printing becomes different from the generated hue in a band or line printed in the backward printing, generating a printed image appearing to be covered with light lateral stripes. However, this problem does not lead to the unacceptable image quality generated in the secondary transfer process. Furthermore, this problem is generated in the high speed printing in which a certain degree of the degradation of image quality can be accepted. For this reason, this problem has not been actively dealt with except for some cases.
According to a method disclosed in Japanese Laid-Open Patent Application No. 11-320926, two printing units are used. The order of colors arranged in one of the printing head units is opposite to the order of colors arranged in the other of the printing head units. One-dot line forward printing by the one of the printing head unit and one dot-line backward printing by the other of the printing head units are formed alternately on a print medium. In this manner, the color difference between the forward printing and the backward printing can be suppressed.
However, in this method, two subsystems for maintaining and recovering functions of the two printing head units are required, resulting in a high cost. Working for cleaning head units becomes two times larger, and a possibility exists that inks are mixed with one another via a wiping blade that cleans nozzle surfaces.
According to a method disclosed in Japanese Published Patent Application No. 7-29423, the forward printing is performed at intervals of every other dot, and the backward printing is performed so as to print the dots that are not printed in the forward printing.
However, in this method, two printing processes, i.e., the forward printing and the backward printing are performed on the same dot line, so that the printing speed does not become higher than the printing speed in the uni- directional printing.